This would unsecure this validator as someone could simply set a own type like "image/blabla" which would then be accepted.
He can set an own type to one of the accepted type (e.g. "image/jpeg"), too.
Additionally is has only effect if headerCheck is activated and no mimetype functions are available.

On the other hand there are some mimetypes which are NO images and still use the image mimetype (image processing programs). These are actually NOT accepted by this validator.
OK, this makes sense. But for example the mimetype "image/jpeg" passes on *.jgw (JPEG world file), too.